The present invention relates to Magnetic Resonance Imaging (MRI) and in particular to a local coil for use with SENSE, SMASH, and other imaging techniques in which the local coil helps distinguish between simultaneously acquired, but separate groups of spins.
In MRI, a uniform magnetic field, B0, is applied to an imaged object along a longitudinal axis or z-axis of a Cartesian coordinate system. The effect of the magnetic field is to align the objects nuclear spins along the z-axis. A radio frequency (RF) excitation signal of the proper frequency oriented in the transverse or x-y plane is then applied to cause the nuclear spins to precess. During a detection stage, this precession is captured as a nuclear magnetic resonance (NMR) signal. Water,because of its relative abundance in biological tissue and the property of its nuclei, is the principal source of such NMR signals in medical imaging.
An image may be generated from the NMR signals by distinguishing among the locations of the source nuclear spins. In a conventional xe2x80x9cslice selectxe2x80x9d imaging sequence, this is be done by limiting the RF excitation to a single slice in the x-y plane. Magnetic gradient fields are then applied along the transverse plane to modify the frequency and phase of the precession of the nuclear spins as a function of their location. A series of RF excitations with different x and y-axis gradient fields provides the data necessary to identify the contribution from nuclear spins in different locations to the NMR signal. The mapping of signal contribution to spin location provides the basis for an MRI image.
The NMR data obtained after each RF excitation provides one line of data in xe2x80x9ck-spacexe2x80x9d. Repetition of the RF excitation with different gradients provides different lines until an area is covered. The k-space area may then be converted to an image. A significant drawback to this other sequential acquisition of lines of k-space data is that the speed of generating in the image is severally limited.
Several techniques have been developed which allow simultaneous acquisition of multiple lines of k-space data from spatially separated regions of the patient. These techniques generally use the spatial information that can be derived from multiple receiving loops placed on each patient which has a different reception pattern for receiving the NMR signal. This additional spatial information allows NMR signals from different locations to be distinguished even though the NMR signals may have the same phase and frequency. Such techniques include Simultaneous Acquisition of Spatial Harmonics (SMASH) and Sensitivity Encoding Technique (SENSE) imaging techniques known to those of skill in the art. These and other techniques that allow simultaneous acquisition of multiple lines of k-space data will henceforth be termed xe2x80x9cparallel acquisition techniquesxe2x80x9d.
Referring now to FIG. 1, a prior art coil 10 suitable for use with parallel acquisition techniques provides a generally cylindrical form 12 aligned with the longitudinal or z-axis. The outer circumference of the cylindrical form supports four, phased array loops 14a to 4d. The loops 14a to 14d may be generally rectangular and in pairwise opposition along the x-axis with each loop 14a to 14d extending approximately 180 degrees about the circumference of the cylindrical form 12. Thus, the first pair of loops 14a and 14b are in opposition at one longitudinal end of the cylindrical form 12 and a second pair of loops 14c and 14d are in opposition about a second end of the form. Each of the loops 14a to 14d provides a separate signal transmission lead 16a to 16d so that NMR signals may be independently obtained from each coil and compared.
Referring now to FIG. 2, the reception patterns 18a to 18d, respectively, of loops 14a to 14d differ along the longitudinal axis (principally in being displaced with respect to one another either transversely or longitudinally) causing each loop 14a to 14d to have different sensitivities to the NMR signal from a given spin, for example, spin 20a. This difference in reception patterns 18 allows parallel acquisition of NMR signals from longitudinally displaced spins 20a and 20b experiencing the same RF excitation and gradient fields and allows for parallel acquisition of spins in up to four separate regions. Such a coil 10 is said to have an acceleration of two in the transverse direction and an acceleration of two in the longitudinal direction.
Considering now only loops 14a and 14c and two longitudinally displaced spins 20a and 20b within the reception patterns 18a and 18c, generally each of leads 16a and 16c will provide an NMR signal that is a combination of signals from spins 20a and 20b. For this simple case, the contributions of each spin 20a and 20b may be separated solving two equations relating the unknowns of contributions to known sensitivities of the two loops 14a and 14c (from their reception patterns 18a and 18b) and known signals from lines 16a and 16b. This process may be expanded for multiple spins and multiple coils using well-known algebraic techniques.
The coil 10, as shown in FIG. 1, may be further accelerated in the transverse plane by shortening the arc length of the loops 14 to of ninety degrees along the circumference of the cylindrical form 12 to provide four rather than two separate transverse regions of sensitivity. Unfortunately, limitations in the number of inputs in current MRI equipment for receiving separate leads 16 prevent the practical use of more than eight loops 14. Thus, in this case, acceleration along the z-axis must be eliminated if a transverse acceleration of four is desired.
The present invention provides a local coil using loops that have reception patterns that are locally sensitive to both longitudinal and transverse displacement of spins. In one embodiment, these loops are triangular. By using loops that can provide either longitudinal or transverse acceleration, a more versatile coil is created. Thus, for example, the present invention can produce in theory a coil providing six transverse acceleration or two longitudinal acceleration requiring only six loops and six leads to the MRI machine.
Specifically, the present invention provides an MRI coil for use in a polarizing longitudinal magnetic field and having at least two loops transversely adjacent across an interface extending in part along the longitudinal axis. The interface is angled with respect to the longitudinal axis. Signal leads attached to the loops separately conduct signals received from each loop to processing circuitry.
Thus, it is one object of the invention to provide a coil with loops that can function for both transverse and longitudinal acceleration.
The two loops may be triangles, for example, right triangle or isosceles triangles.
Thus, it is another object of the invention to provide a loop shape that may be simply fabricated and designed.
The triangles may tile to fill a rectangular area.
Thus, it is another object of the invention to provide a loop shape readily adaptable to the rectangular surfaces of unwrapped cylinders or planes commonly used in local coils.
The loops may conform generally to the surface of a cylinder having an axis of radial symmetry parallel to the longitudinal axis or may conform to the surface of a plane parallel to the longitudinal axis or may conform to opposed surfaces of a rectangular prism extending along the longitudinal axis.
Thus, it is another object of the invention to provide a design adaptable to a wide variety of common coil topologies.

These particular objects and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention.